Field of the Invention
The present invention relates to a photoacoustic apparatus.
Description of the Related Art
Researches on optical imaging techniques that irradiate a living body with light emitted from a light source such as a laser and reconstruct, as an image, information on the inside of the living body obtained based on the incident light have progressed actively in a medical field. A photoacoustic tomography (PAT) is one of the optical imaging techniques. According to the photoacoustic imaging technique, an object is irradiated with pulsed light generated from a light source. The energy of the pulsed light propagated and diffused in the living body is absorbed in the tissues of the living body and acoustic waves (typically ultrasound waves) are generated. The generated acoustic waves propagate through the living body and are received by a probe and detected as electrical signals. An information processing device performs a mathematical analysis process (an image reconstruction process) on the obtained signals to visualize information related to optical characteristic values inside an object. In this way, information on the inside of the object (for example, an initial acoustic pressure, an optical characteristic value (particularly, optical energy absorption density and absorption coefficient), and a distribution thereof) can be obtained. The obtained information can be used for specifying a distribution of absorbers in the living body, the location of malignant tumors, and the like.
In photoacoustic measurement, initial acoustic pressure P0 of acoustic waves generated from a light absorber in an object can be expressed by the following equation.P0=Γ·μa·Φ  (A)Here, Γ is a Gruneisen coefficient which is a division of the product of a volume expansion coefficient β and the square of the speed of sound c by the specific heat capacity Cp at constant pressure. It is known that Γ takes an almost constant value if the object is determined. μa is an absorption coefficient of the light absorber, and Φ is a light quantity at the position of the light absorber (a quantity of light radiated to the light absorber; also referred to as light fluence). The initial acoustic pressure P0 generated by the light absorber inside an object propagates through the object as acoustic waves which are detected by a probe disposed on the surface of the object. A change with time in the acoustic pressure of the detected acoustic waves is measured, and the initial acoustic pressure distribution P0 can be calculated from the measurement results using an image reconstruction method such as a back-projection method. When the calculated initial acoustic pressure distribution P0 is divided by the Gruneisen coefficient Γ, a distribution of the product of μa and Φ (that is, an optical energy density distribution) can be obtained. Moreover, if a light quantity distribution Φ inside an object is known, the absorption coefficient distribution μa can be obtained by dividing the optical energy density distribution by the light quantity distribution Φ. That is, when a light absorber having known optical characteristics in relation to light radiated to the inside of an object is injected as a contrast agent, acoustic waves corresponding the abundance of the contrast agent can be acquired.
In photoacoustic measurement, pulsed light generated from a light source is radiated to an object. Acoustic waves are generated from a sound source that absorbed the energy of light propagated and diffused inside the object. The generated acoustic waves are detected by a probe disposed around the object. A spatial distribution image of the sound source is obtained by reconstructing the detected acoustic waves. In this case, in order to guarantee the accuracy of the image, a series of operations of irradiating an object with pulsed light generated from a light source and receiving photoacoustic waves propagating from the object may be performed repeatedly. Thus, in this case, a certain period is required until the last acoustic waves are received after light irradiation for photoacoustic measurement is started and acoustic waves are received. That is, a time lag occurs between the first photoacoustic measurement (the first light irradiation and the first acoustic wave reception) and the last photoacoustic measurement.
On the other hand, when a living body is an object, the blood (hemoglobin) is one of the substances which have highly absorptive to near-infrared light. That is, information on the spatial distribution of the blood can be obtained by performing photoacoustic measurement on the living body using near-infrared light. However, sufficiently high S/N ratio of signals cannot be guaranteed by the abundance of hemoglobin only, and sufficiently high contrast may not be obtained. In this case, when a light absorber is further injected into the blood vessel as a contrast agent, a PAT image (also referred to as a photoacoustic image) in which the contrast is enhanced depending on the density of a light absorber present in a blood vessel portion can be obtained. As conditions of a contrast agent usable in a living body, the contrast agent is required to have such optical characteristic that the contrast agent can better absorb light in a wavelength region in which light is highly transmissive in a living body. Indocyanine green (hereinafter sometimes abbreviated as ICG) can be ideally used as an example of the contrast agent that satisfies such conditions. Since ICG is a material approved for medical use and is a light absorber that can better absorb light in a near-infrared wavelength region having a high transmissive property in a living body, ICG can be used as a preferable contrast agent for photoacoustic measurement of living bodies. In Optics Letters, Vol. 29, Issue 7, pp. 730-732 (2004), ICG was injected into a mouse to investigate the retention of ICG in the blood.
However, it is known that ICG is rapidly cleared from the blood. Specifically, it is reported that the half-life of ICG in the blood is approximately 3 minutes (J Clin Invest., 1960 April; 39(4): p. 592-600). This means that, when ICG is used as a vascular contrast agent, the period in which ICG can function in the blood vessel is very short. Thus, the time lag that occurs means that, even if photoacoustic measurement started immediately after injection, a blood concentration of ICG decreases greatly in the period between the first acoustic wave acquisition time and the last signal acquisition time.
On the other hand, the strength of acoustic waves measured by a probe in angiography depends on the blood ICG concentration as in Equation A. Thus, when a light absorber such as ICG of which the blood concentration changes during measurement, a variation occurs in the acoustic waves received based on the period elapsed from injection into the blood and the accuracy of photoacoustic image decreases.
In view of the above problems, it is an object of the present invention to provide a photoacoustic apparatus capable of reducing the influence of a strength variation in acoustic waves based on a measurement period in photoacoustic measurement which uses a contrast agent.